Several methods and systems are known for controlling the sampling time in modem receivers which utilize equalizers with a tap spacing equal to the modulation interval T, or a fraction thereof. The known methods and systems were disclosed, e.g., in the following publications and patent:
(a) D.L. Lyon: "Timing Recovery in Synchronous Equalized Data Communication", IEEE Transactions on Communications, Vol. COM-23 (1975) pp. 269-274.
(b) F. G. Caron et al.: U.S. Pat. No. 4,039,748 "Method and Device for Synchronizing the Receiver Clock in a Data Transmission System".
(c) D. Godard: "Passband Timing Recovery in an All-Digital Modem Receiver", IEEE Transactions on Communications, Vol. COM-26 (1978) pp. 517-523.
(d) G. Ungerboeck: "Fractional Tap-Spacing Equalizer and Consequences for Clock Recovery in Data Modems", IEEE Transactions on Communications, Vol. COM-24 (1976) pp. 856-864.
(e) P. R. Chevillat, D. Maiwald, G. Ungerboeck: "Rapid Training of a Voice-Band Data-Modem Receiver Employing an Equalizer with Fractional-T Spaced Coefficients", IEEE Transactions on Communications, Vol. COM-35 (1987) pp. 869-876.
Three of these disclosures (a, b and c) describe schemes which employ bandpass filters to extract signal components from the bandedges of the received signal for timing control ("bandedge timing"). The average energy of the sum of the bandedge signals is a periodic function, with period T, of the sampling phase at which the bandedge signals are observed. This dependency is exploited for timing-phase control. In particular, it is argued that the sampling phase should be adjusted to a value for which the maximum bandedge energy is obtained. Equalizers with T-spaced taps achieve optimum performance for this phase.
The known schemes adjust the sampling phase only to this phase, and later maintain it at this value. The selectivity of the bandpass filters plays an important role. Furthermore, the signal power in the bandedge regions, which depends on the a priori unknown attenuation characteristics of the currently used transmission channel, acts as a multiplicative factor in the obtained phase-error measurements, and thus influences the dynamic behavior of the timing control scheme. It is desireable to have a timing control scheme which is independent of these conditions. For receivers with an equalizer, whose taps are spaced by a fraction of T (FTS equalizer), it is furthermore advantageous to have a timing control scheme which only measures the initial random sampling phase and then maintains the sampling phase at this value.